Optical waveguide made of polymer material and a method of fabricating the same

ABSTRACT

An optical waveguide having a clad and a core, the core being made of polymer material containing a repetitive unit having formula (1), (2) or (3):                    
     Each of these polymer materials has a higher glass transition temperature and lower water absorption than those of deuterated PMMA, has a transparency equivalent with that of deuterated PMMA, and shows neither light absorption nor scattering in the operating wavelength region. An optical waveguide with a core fabricated using these polymer materials is high in heat resistance and low in water absorption. Thus using the waveguide will successfully provide optical communication elements with an advanced durability against the environment.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to polymer material used for composingoptical communication parts, optical waveguides using such polymermaterials and a method of fabricating the optical waveguides.

2. Description of the Background Art

In the field of telecommunications, development of optical waveguide hasbeen recognized as a critical issue to enable large capacitycommunications.

Prerequisites to the materials used for composing optical communicationparts such as an optical waveguide include higher transparency atwavelengths in the near-infrared range to which the wavelength ofoptical signals belongs and less scattering. The materials are alsorequired to have controllability in their refractive indices since theyare used to compose optical transmission paths.

Glass or other inorganic crystalline materials have conventionally beenused as materials for composing optical communication parts such as anoptical waveguide. These materials, however, suffer from theirexpensiveness and difficulty in processing.

In recent years, polymer materials, such as PMMA (polymethylmethacrylate) and PS (polystyrene), became more popular thanks to theirinexpensiveness and easier processing as compared with those of glass orother inorganic crystalline materials. Use of such material can providea film-type optical waveguide with wider area and higher flexibilitythan the conventionals. It becomes also possible to obtain a functionaloptical waveguide by introducing functional compounds or functionalgroups into such polymer materials.

Fabricating such an optical waveguide essentially requires a method ofprocessing the polymer materials into a desired form. Typical of such amethod has been the reactive ion etching (RIE) method using oxygenplasma. The fabrication process of a polymer-made optical waveguide bythe RIE method has to be proceeded as generally shown in FIGS. 5A to 5E.Here, FIGS. 5A to 5E show schematic cross-sectional views useful forunderstanding the major steps in sequence for fabricating a polymer-madeoptical waveguide using the RIE method.

First, on a base 101, a polymer film 103 a as an underclad, a polymerfilm 103 b for forming a core, and a photoresist film 105 for forming anetching mask are formed in this order, FIG. 5A.

To obtain the etching mask corresponded to a desired patterned shape byprocessing the photoresist film 105, the photoresist film 105 is thensubjected to selective light exposure through a photomask 107, FIG. 5B,corresponding to the patterned shape. This results in forming a latentimage of the pattern in the photoresist film 105. The photoresist film105 after exposed with the light is then developed to obtain a resistpattern 105 x, FIG. 5C. The example shown here relates to a case withnegative photoresist.

RIE with an oxygen-base etching gas is then carried out using the resistpattern 105 x as an etchingmask 105 x, and aportion of the polymer film103 b being exposed from the etching mask 105 x is removed. A core 103 xmade of the residual portion of the polymer film 103 b is thus formed onthe underclad 103 a, FIG. 5D.

On the specimen on which the core 103 x has been formed, a polymer film111 for forming overclad is formed to obtain an optical waveguide 113,FIG. 5E. The overclad 111 can be formed by, for example, coating on thespecimen a coating fluid containing material of the overcdad, and isthen allowed to dry.

As for PMMA and PS, some approaches have been taken to improvetransparency in the near-infrared region. More specifically, thesematerials show absorption ascribable to C—H bonds in their molecules inthe near-infrared region, and thus deuterated PMMA, that is, PMMA whosehydrogen atoms are substituted with deuterium atoms has been developed.Deuterated PMMA shows absorption in the far-infrared region as shiftedfrom the near-infrared region.

The above-described PMMA, PS and deuterated PMMA composing the core ofthe optical waveguide, however, are low in glass transition temperature.For instance, both of the PMMA and deuterated PMMA have a glasstransition temperature of 107° C., so that these materials may easily besoftened due to heat treatment such as soldering, if they are used tocompose electronic parts for computers or so.

These materials also suffer from relatively high water absorption. Bothof the PMMA and deuterated PMMA have a value of water absorption as highas 2.0%. The materials composing optical communication parts may altertheir refractive indices due to water absorption, which may causeundesirable transmission error in optical communications.

The PS further has a specific problem on birefringence. In theconventional fabrication process of optical waveguides based on the RIEmethod, a number of steps and a long process time are necessary forforming the pattern, as is clear from the description referring to FIGS.5A to 5E. Problems also reside in that apparatus used for the RIE methodcosts high and requires special skills in the operation.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide polymermaterial superior in transparency and free from scattering in theoperating wavelength region.

It is another object of the present invention to provide an opticalwaveguide having a core made of material with higher heat resistance,lower water absorption and no birefringence.

It is still another object of the present invention to provide anoptical waveguide simpler in structure and easier in fabrication processwithout using the RIE method.

The inventors of the present invention, after extensive studies, focusedon the fact that imidated polymer material becomes higher in glasstransition temperature and lower in water absorption as compared withthose of the original material before imidation, which has led to thepresent invention.

A polymer material according to the present invention contains arepetitive unit having formula (1):

The material expressed by the formula (1) can be obtained by, forexample, reacting deuterated PMMA having superior transparency in thenear-infrared region with deuterated methylamine, where deuteratedmethylamine reacts with the ester bond portion of deuterated PMMA toeffect intramolecular imidation. The resultant imide has a cyclicstructure. The imidated deuterated PMMA, i.e. deuterated polymethylmethacrylimide, has a higher glass transition temperature and a lowerwater absorption as compared with those of deuterated PMMA, and has atransparency equivalent with that of deuterated PMMA. The material isthus favorable as the one for optical communication parts.

The present invention also claims a polymer material containing arepetitive unit having formula (2):

The material expressed by the formula (2) can be obtained by, forexample, reacting deuterated PMMA with ethylenediamine, whereetylenediamine reacts with the ester bond portion of deuterated PMMA.One repetitive unit and one amino group react each other. Since oneethylenediamine molecule has two amino groups, two repetitive units ofdeuterated PMMA and one ethylenediamine molecule can react. Thus theproducts (polymer material) of the reaction will have a structure inwhich two repetitive units of deuterated PMMA are crosslinked withethylenediamine. The repetitive unit of this polymer material has twocyclic portions each of which being similar to the above-describeddeuterated polymethyl methacrylimide. Also this polymer material has ahigher glass transition temperature and a lower water absorption ascompared with those of deuterated PMMA, and has a transparencyequivalent with that of deuterated PMMA. The material is thus favorableas the one for optical communication parts.

An optical waveguide of the present invention comprises a clad and acore, and the core is made of polymer material containing a repetitiveunit having formula (1), (2) or (3):

A polymer material, e.g. polydimethyl glutarimide (PMGI) expressed bythe formula (3), can be obtained by, for example, reacting PMMA withmethylamine, where methylamine reacts with the ester bond portion ofPMMA to effect intramolecular imidation. The resultant imide has acyclic structure. The imidated PMMA, i.e. PMGI, has a higher glasstransition temperature and a lower water absorption as compared withthose of PMMA, and shows neither light absorption nor scattering in theoperating wavelength region. There is no problem on birefringence. Thematerial is thus favorable as the one for forming the core.

Molecular weights of the polymer materials expressed with formulae (1),(2) and (3) can be selected to arbitrary values according to objects oftheir use. If these polymer materials to be formed into film by the spincoating method, it is preferable to adjust their degrees ofpolymerization so that their molecular weights fall within a range from12,500 to 540,000, and more preferably from 150,000 to 540,000. Theviscosity of coating fluid to be coated depends on the evaporation rateof solvent used to dissolve these polymer materials. The viscosity ofthe coating fluid is thus adjusted based on the molecular weights ofthese polymer materials and their amounts to be dissolved in thesolvent. More specifically, it is preferable to adjust the viscosity ofthe coating fluid between 100 cP and 10,000 cP. The molecular weights ofthese polymer materials between 150,000 and 540,000 will facilitate theadjustment of the viscosity of the coating fluid, and polymer film withsmooth surface will be obtained.

An optical waveguide with a core fabricated using these polymermaterials is high in heat resistance and low in water absorption. Thususing the optical waveguide will successfully provide opticalcommunication parts with an advanced durability against the environment.

In the optical waveguide of the present invention, the clad ispreferably made of a material having a smaller refractive index thanthose of the above polymer materials. PMMA, for example, isrecommendable as a material for the clad.

According to another constitution of the optical waveguide of thepresent invention, the optical waveguide comprises a substrate made ofinorganic material; a core formed on the substrate; and an overclad madeof polymer and formed on the substrate to cover the core; refractiveindices of the substrate and the overclad being approximately the same,and the ratio (n₁-n₂)/n₁ of the difference of the refractive index n₁ ofthe core from that n₂ of the substrate or overclad to the refractiveindex n₁ of the core being within a range from 0.3 to 3.0%.

This allows the substrate to be used as an underclad (lower clad) in theconventional sense and provides an optical waveguide with a simplerstructure. Since the ratio of the difference in refractive index betweenthe core and the substrate or overclad to the refractive index of thecore falls within a range from 0.3 to 3.0%, light used fortelecommunications can successfully be transmitted over the opticalwaveguide based on total reflection.

Glass, being widely used as a substrate material, is recommendable as aninorganic material composing the substrate of the optical waveguide.Polymer composing the overclad can be of a refractive index whichmatches well with that of the substrate, and is exemplified as UV (ultraviolet) curing or setting resin, thermosetting resin and so forth.

The inorganic material of the substrate and the polymer of the overcladare selected so that their refractive indices approximately coincidewith each other. When barium borosilicate glass is used as the inorganicmaterial, for example, an UV setting resin (manufactured by NTT AdvancedTechnology Corporation, product code No. 8101) is typically used as apolymer since it has a refractive index substantially equal to 1.528.

A typical optical waveguide comprises a substrate made of bariumborosilicate glass; a core made of PMGI and formed on the substrate; andan overclad made of UV setting resin and formed on the substrate so asto cover the core; thus allowing the ratio of the difference inrefractive index between the core and the substrate or overclad to therefractive index of the core to fall within a range from 0.3 to 3.0%.

According to the method of fabricating an optical waveguide of thepresent invention, the optical waveguide comprising a core on a base isformed by the steps of: forming on the base a core-forming materiallayer with a positive resist property; subjecting the core-formingmaterial layer to light exposure through a photomask which shadows acore-formative area of the core-forming material layer; and, developingthe core-forming material layer after exposed by dipping it intodeveloping solution to leave a portion of the core-forming materiallayer corresponding to the core-formative area.

By contrast to the conventional process of forming a core on a base, inwhich a core-forming material layer is formed by coating on a base, aresist pattern is then formed by photolithography, and the core-formingmaterial layer is etched by the RIE method using the resist pattern asan etching mask, the present invention allows core forming only by thesteps of forming the core-forming material layer on the base and ofeffecting light exposure and development process to the core-formingmaterial layer. Thus an optical waveguide can be fabricated quite easilywith a lesser number of process steps without using the RIE method.

It is preferable to use, as a core-forming material with a positivephotoresist property, PMGI containing a repetitive unit expressed byformula (3). This material is known to cleave at three bonds within asingle repetitive unit when irradiated by energy beam with a wavelengthof 300 nm or shorter (deep UV light, for example). Formula (4) simplyexpresses this reaction.

The light exposed area of the core-forming material layer becomessoluble to developing solution due to the bond cleavage as defined byformula (4).

When developed, the exposed area dissolves to the developing solution tobe removed, while the core-formative portion remains. Thus a core can befabricated in a manner quite easier than in the conventional process.

A base in a context of the present invention typically refers to anunderclad (lower clad) formed on an arbitrary base member, and asubstrate (including that in a form of film) available as an underclad.

The above-described arbitrary base member for forming the undercladrefers to an base member arbitrarily selected depending on a design ofthe optical waveguide, and either a base member made of inorganic ororganic material is available. More specifically, it can be selectedfrom a semiconductor substrate made of silicon or compoundsemiconductor; a glass substrate; a ceramic substrate; and base materialmade of arbitrary polymer material other than that used for theunderclad. The arbitrary base member can, of course, be an intermediateproduct in which other electronic parts or optical parts are alreadyincorporated.

A substrate available as an underclad is, for example, made of amaterial having a refractive index approximately the same with that ofthe overclad. A substrate made or glass or other inorganic materials istypically available.

A polymer material as a core-forming material preferably has a molecularweight within a range from 12,500 to 540,000.

When the spin coating method is adopted to form the core-formingmaterial layer on the base, the viscosity of a coating fluid to becoated on the base will preferably be within a range from 100 cP to10,000 cP. Using the coating fluid having a viscosity within the aboverange allows a desired thickness of a layer to be formed. It becomesalso possible to smoothen the surface of the layer formed by the spincoating method. The viscosity of the coating fluid is adjusted based onthe molecular weight of the polymer material and the amount of thepolymer material to be dissolved in the solvent. It is thus preferableto select the molecular weight of the polymer material between 150,000and 540,000 to adjust the viscosity of the coating fluid within theabove range. The molecular weight is more preferably selected within arange from 150,000 to 540,000.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the present invention will become moreapparent from consideration of the following detailed description takenin conjunction with the accompanying drawings in which:

FIGS. 1A to 1D are schematic process diagrams useful for understandingan embodiment of the present invention;

FIGS. 2A to 2D are schematic process diagrams, as continued from FIGS.1A to 1D, useful for understanding further processes of the embodiment;

FIG. 3 is a schematic perspective view showing an optical waveguide towhich the present invention is applied;

FIGS. 4A to 4D are schematic process diagrams, like FIGS. 1A to 1D,useful for understanding another embodiment of the present invention;and

FIGS. 5A to 5E are schematic process diagrams, like FIGS. 1A go 1D,useful for understanding a conventional process for fabricating anoptical waveguide.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the polymer material, optical waveguide and amethod of fabricating thereof in accordance with the present inventionwill be described hereinafter. Materials, equipment, consumption of thematerials, numerical conditions including temperature and pressureadopted in the fabrication processes are mare examples preferred withina scope of the present invention. The polymer materials, therefore, arenot limited to those prepared using the conditions described below.

First Embodiment

In this first embodiment, a polymer material containing a repetitiveunit having formula (1) is prepared as described below.

Ten grams of deuterated polymethyl methacrylate (deuterated PMMA,product of Sowa Kagaku Co., Ltd.; molecular weight 137,700; product codeNo. P818) as a source of the polymer material is added to 90 g oftetrahydrofuran (THF) as a solvent. The mixture is stirred at roomtemperature for 24 hours to ensure thorough solubilization. The obtainedsolution is denoted as a first solution. A second solution is separatelyprepared by dissolving deuterated methylamine (product of Kanto KagakuCo., Ltd.) in methanol to obtain a 50 wt % solution. Twenty grams eachof the first and second solutions are then mixed and allowed to react at260° C. for 2 hours.

It is supposed that imidation as defined by the following equation (5)proceeds during the reaction:

where n represents the degree of polymerization, and can be an integerof unity or larger.

After the reaction, the reaction mixture is allowed to stand for naturalcooling to the room temperature, which is followed by addition of 500 mlof pure water. A product supposed to be the polymer material containingthe repetitive unit expressed by formula (1) deposits. That is, apolymer material expressed by formula (5a) on the right side of thereaction equation (5) is successfully synthesized. The product is washedwith water and dried.

To confirm whether the product is a real polymer material expressed byformula (1) (more particularly, by formula (5a)), the obtained productis analyzed by infrared spectroscopy. For this, the product is melted at260° C. to be molded into a pellet, and is then subjected to themeasurement.

Absorption spectra characteristic to methacrylimide were observed atwave numbers of 1720 cm⁻¹, 1663 cm⁻¹ and 750 cm⁻¹, which provedimidation.

The glass transition temperature of this polymer material was measuredas 165° C., which was found to be significantly raised since the glasstransition temperature of the conventional deuterated PMMA is 107° C.Thus heat resistance of the polymer material can be improved.

The pellet of the polymer material was also subjected to the measurementof water absorption, in which the pellet was allowed to stand in a hotand humid environment with a humidity of 90% and a temperature of 90° C.for 100 hours before the measurement. Water absorption of the pellet was0.9%, which was found to be lowered since the water absorption ofdeuterated PMMA is 2.0%.

A measurement of the refractive index of the polymer material gave avalue of 1.51, which was slightly higher than that of deuterated PMMA.The refractive indices measured for the TE (transverse electric) modeand TM (transverse magnetic) mode, both of which are polarization modes,were the same. It was thus indicated that the material does not showbirefringence.

In the following paragraphs, an example of fabrication processes for anoptical waveguide using the polymer material of formula (1) obtainedabove will be described referring to FIGS. 1A to 1D and FIGS. 2A to 2D.The figures referred to in this specification show nothing but aschematic expression of shape, dimension and arrangement of individualcomponents so as to help understanding. In these figures, the likecomponents will follow the same reference numerals, and their repetitivedescription may be omitted.

FIGS. 1A to 1D are process diagrams useful for understanding one typicalembodiment for fabricating an optical waveguide using the polymermaterial of the present invention. Those figures depict the states ofthe specimen with its cross-sectional end at the major steps of theprocess. FIGS. 2A to 2D are schematic process diagrams as continued fromFIGS. 1A to 1D.

The structure of the optical waveguide includes an underclad provided ona substrate, a linear core provided on the underclad, and an overcladprovided so as to cover the core. In this embodiment, the opticalwaveguide will be fabricated according to steps [1] to [3] as describedbelow.

[1] The underclad is formed on the substrate.

Three grams of polymethyl methacrylate (PMMA, product of Sowa KagakuCo., Ltd.; molecular weight 75,000; product code No. 307), as an exampleof a material for the underclad whose refractive index is smaller thanthat for deuterated polymethyl methacrylimide expressed by formula (1),is added to 7 g of 2-methoxyethyl acetate as a solvent. The mixture isstirred at room temperature for 12 hours to ensure thoroughsolubilization, and then filtrated through a filter (pore size 0.45 μm,PTFE membrane), to prepare a 30 wt % coating fluid of PMMA.

The coating fluid is then spin-coated on a silicon substrate 11 at 3,000rpm for 30 seconds. The obtained coated film is then dried at 95° C. for6 hours in a furnace conditioned to the atmospheric environment. A PMMAfilm of approx. 7 μm thick as an underclad 13 is thus formed on thesilicon substrate 11, FIG. 1A.

[2] Next, a linear core is formed on the underclad.

Two grams of deuterated polymethyl methacrylimide of formula (1)obtained above is added to 8 g of 2-methoxyethyl acetate as a solvent.The mixture is stirred at room temperature for 12 hours to ensurethorough solubilization, and then filtrated through a filter (pore size0.45 μm, PTFE membrane), to prepare a 20 wt % coating fluid ofdeuterated polymethyl methacrylimide.

The coating fluid is then spin-coated on the underclad 13 at 3,000 rpmfor 30 seconds. The obtained coated film is then dried at 95° C. for 6hours in a furnace conditioned to the atmospheric environment. Adeuterated polymethyl methacrylimide layer 15 of approx. 4 μm thick isthus formed on the underclad 13, FIG. 1B.

A silicon-containing resist (DLR (trade name): product of NTT AdvancedTechnology Corporation) is then spin-coated on the deuterated polymethylmethacrylimide layer 15 at 3,000 rpm for 30 seconds. The obtained coatedfilm is then dried at 100° C. for 20 minutes in a furnace conditioned tothe atmospheric environment, to form a resist layer 17, FIG. 1C. Aphotomask 21 having a linear pattern 19 is set above the resist layer17, the pattern being a 10-μm-wide line-and-space. Using the photomask21, light is exposed for 10 seconds using an exposure apparatus calledmask aligner, FIG. 1D. Light exposure is graphically expressed with thearrows in FIG. 1D. This process is continuously followed by adevelopment process. The resist layer 17 is thus transformed into alinear resist pattern 17 x, FIG. 2A. Etching with aid of oxygen plasmais then carried out to remove the portion of the deuterated polymethylmethacrylimide layer 15 not covered with the resist pattern 17 x, FIG.2B. The remaining resist pattern 17 x is then removed with a strippingsolution, FIG. 2C. A core 15 x, having a 10-μm-wide line-and-spacepattern and made of deuterated polymethyl methacrylimide, is thus formedon the underclad 13, FIG. 2C.

[3] Next, an overclad is formed so as to cover the core.

Similarly to the forming process of the underclad as described inprocess [1], a 30 wt % coating fluid of PMMA is prepared. The coatingfluid is then coated on the underclad 13 so as to cover the core 15 x.Here, the spin coating method is adopted and coating is carried out at3000 rpm for 30 seconds. The obtained coated film is then dried at 95°C. for 6 hours in a furnace conditioned to the atmospheric environment.An overclad 23 of approx. 7 μm thick is thus formed over the core 15 x,FIG. 2D.

The obtained laminate 25 is then polished on its both side facesincluding the both end surface of the linear core 15 x to complete anoptical waveguide.

According to this embodiment, an optical waveguide in which the core 15x being made of deuterated polymethyl methacrylimide, and the underclad13 and overclad 23 being made of polymethyl methacrylate is fabricated.Thus obtained optical waveguide was input with near-infrared light of1,330 nm, which is used in optical communications, to measure thetransmissive attenuation. It was found that a substantially constantvalue (approx. 0.4 dB/cm) was obtained.

Second Embodiment

A polymer material containing a repetitive unit having formula (2) isprepared, for example, as described below.

With reference to the instant, second embodiment, an example of forminga film of the polymer material expressed by the above formula (2) willbe described. Three grams of deuterated PMMA (product of Sowa KagakuCo., Ltd.; molecular weight 137,700; product code No. P818) as a sourceof the polymer material is added to 7.0 g of 2-methoxyethyl acetate as asolvent. The mixture is stirred at room temperature for 12 hours toensure thorough solubilization, and then filtrated through a filter(pore size 0.45 μm, PTFE membrane), to prepare a 30 wt % coating fluidof deuterated PMMA.

The coating fluid is then spin-coated at 3,000 rpm for 30 seconds on aglass substrate with a refractive index of 1.459. The obtained coatedfilm is then dried at 95° C. for 6 hours in a furnace conditioned to theatmospheric environment. A deuterated PMMA film of approx. 7 μm thick isthus formed on the glass substrate.

The glass substrate along with the deuterated PMMA film formed thereonare then dipped in a 20 wt % ethylenediamine (Kanto Kagaku Co., Ltd.)solution in methanol for 12 hours. The temperature of the solution isthen raised to 300° C. while keeping the substrate dipped therein, andkept at this temperature for 2 hours.

It is supposed that crosslinking as defined by the following formula (6)proceeds during the reaction:

where n represents the degree of polymerization, and is an integer ofunity or larger. After the reaction, the film is washed with water anddried.

The film made of the polymer material containing the repetitive unitexpressed by formula (2) is thus formed. That is, a polymer materialexpressed by formula (6a) on the right side of the reaction equation (6)is successfully synthesized.

Thus obtained film of the polymer material was input with near-infraredlight of 1,330 nm, which is used in optical communications, to measurethe transmissive attenuation. It was found that an substantiallyconstant value (approx. 0.4 dB/cm) was obtained independent oflocations.

A glass transition temperature of the film was measured as 180° C. bythermal analysis. This is by 70° C. or more and higher than the glasstransition temperature of the conventional deuterated PMMA, proving thatthe film has a superior heat resistance.

The water absorption of the film, measured in a similar manner to thefirst embodiment, was 0.5%, which was one-fourth of that for deuteratedPMMA. Also water absorption was thus successfully reduced.

In order to fabricate an optical waveguide having the core 15 x, whichis made of material defined by formula (2), the same processes mayadvantageously be applicable except those for preparing the core 15 x.More specifically, the core 15 x is formed by the processes describedwith reference to the second embodiment, instead of the step [2]described with reference to the first embodiment. The clad including theunderclad 13 and the overclad 23 may be formed by the steps [1] and [3]described on the first embodiment.

The polymer material containing the repetitive unit expressed by formula(2) (more specifically, the polymer material expressed by formula (6a))can be prepared by a method not limited to that described with referenceto the second embodiment but the steps described with reference to thefirst embodiment, for example.

More specifically, the second solution in the first embodiment can bereplaced with a 20 wt % ethylenediamine solution in methanol, with theremaining features such as processes and materials being the same as thefirst embodiment. It can easily be confirmed by the infrared absorptionspectrum whether or not the polymer material thus obtained is really apolymer material expressed by formula (6a).

Third Embodiment

Polydimethyl glutarimide (PMGI) containing a repetitive unit havingformula (3), a kind of polymer material, is prepared as described below.

Ten grams of PMMA (product of Sowa Kagaku Co., Ltd.; molecular weight540,000; product code No. 037D) as a source of PMGI is added to 90 g oftetrahydrofuran (THF) as a solvent. The mixture is stirred at roomtemperature for 24 hours to ensure thorough solubilization. The obtainedsolution is denoted as a first solution.

A second solution is separately prepared by dissolving methylamine(product of Kanto Kagaku Co., Ltd.) in methanol to obtain a 50 wt %solution. The solvent for dissolving methylamine is not limited tomethanol, but any kind of solvent is available as far as it does notinhibit the reaction between PMMA and methylamine. Twenty grams each ofthe first and second solutions are then mixed and allowed to react at260° C. for 2 hours in an autoclave.

It is supposed that imidation as expressed by the following equation (7)proceeds during the reaction:

where n represents the degree of polymerization, and can be an integerof unity or larger.

After the reaction, the reaction mixture is allowed to stand for naturalcooling to the room temperature, which is followed by addition of 500 mlof pure water. A product supposed to be PMGI containing the repetitiveunit expressed by formula (3) deposits. That is, PMGI expressed byformula (7a) on the right side of the reaction equation (7) issuccessfully synthesized. The product is washed with water and dried.

To confirm whether the product is a real PMGI expressed by formula (3)(more particularly, by formula (7a)), the obtained product is analyzedby infrared spectroscopy. For this, the product is melted at 260° C. tobe molded into a pellet, and was then subjected to the measurement.

The absorption spectra characteristic to methacrylimide were observed atwave numbers of 1720 cm⁻¹, 1663 cm⁻¹ and 750 cm⁻¹, which proved that theproduct was PMGI.

The glass transition temperature of this polymer material was measuredas 195° C., which was found to be significantly raised since the glasstransition temperature of the conventional deuterated PMMA is 107° C.Thus heat resistance of the polymer material can be improved.

The pellet of the polymer material was also subjected to the measurementof water absorption, in which the pellet was allowed to stand in a hotand humid environment with a humidity of 90% and a temperature of 90° C.for 100 hours before the measurement. The water absorption of the pelletwas 0.9%, which was found to be lowered since the water absorption ofdeuterated PMMA is 2.0%.

The measurement of the refractive index of the polymer material gave avalue of 1.53, which was slightly higher than that of deuterated PMMA.The refractive indices measured for the TE (transverse electric) modeand TM (transverse magnetic) mode, both of which are polarization modes,were the same. It was thus indicated that the material does not showbirefringence.

Next, an example of fabricating an optical waveguide using PMGI havingformula (3) prepared above will be described referring to FIG. 3 andFIGS. 4A to 4D. FIG. 3 is a schematic perspective view of an opticalwaveguide of the present invention. FIGS. 4A to 4D are process diagramsuseful for understanding a fabrication process of an optical waveguideof the present invention. FIGS. 4A to 4D respectively indicate thestates of the specimen with its cross-sectional end at the major step ofthe process.

Referring to FIG. 3, an optical waveguide 30 comprises a substrate 32made of inorganic material; a core 34 formed linearly on the substrate32; and an overclad 36 made of polymer and formed on the substrate 32 soas to cover the core 34; refractive indices of the substrate 32 and theoverclad 36 being approximately the same. In this embodiment, a glasssubstrate is selected as the substrate 32, where the glass substratebeing in particular a barium borosilicate glass substrate with arefractive index of 1.528. An UV setting resin is selected as apolymerfor composing the overclad 36, where the polymer being in particular anUV setting resin with a refractive index of 1.528 (product ofNTT-Advanced Technology Corporation, product code No. 8101).

The core 34 is made of a material such that the relative difference, orthe ratio (n₁-n₂)/n₁ of the difference of the refractive index n₁ of thecore from that n₂ of the substrate or the overclad to the refractiveindex n₁ of the core, falls within a range from 0.3 to 3.0%. Here thePMGI prepared above is used as a material composing the core 34.

In this embodiment, the optical waveguide 30 will be fabricatedaccording to the following steps [1] to [4]:

[1] A core-forming material layer 38 is formed on the substrate 32.

A barium borosilicate glass substrate (refractive index 1.528, productof Corning Incorporated, product code No. 7059) is procured as thesubstrate 32.

Next, 2.0 g of PMGI prepared above is added to 8.0 g of a solventcomprising a 5:1 mixture of cyclopentanone (product of Kanto Kagaku Co.,Ltd.) and tetrahydrofurfuryl alcohol (product of Kanto Kagaku Co.,Ltd.). The mixture is stirred at 60° C. for 12 hours to ensure thoroughsolubilization. The solvent for PMGI is not limited to that describedabove, and available solvents include THF, DMAC (dimethylacetamide),methyl acetate, ethyl acetate and so forth.

The mixture is then filtrated through a filter (pore size 0.45 μm, PTFEmembrane, product of Toyo Roshi Kaisha Ltd.), by which insoluble part ofPMGI is filtered off. The filtrate is obtained as a coating fluid ofPMGI at a concentration of approx. 20 wt %.

The thus prepared coating fluid is then spin-coated on the substrate 32made of barium borosilicate glass. Here, for example, approx. 5 g of thePMGI coating fluid is placed on a 3-inch-square substrate 32 (where 1inch equals to approx. 2.54 cm) and the substrate 32 is thenpreliminarily rotated at 1,000 rpm for 10 seconds. The rotation speed isgradually raised in the next 20 seconds up to 3,000 rpm, and this speedis kept for 30 seconds for main rotation. The rotation of the substrate32 is then gradually slowed down to stop in the next 10 seconds.

The substrate 32 on which a PMGI coated film is formed is then heated todryness for 5 minutes at 250° C. on a hot plate in the atmosphericenvironment. A PMGI layer of approx. 5 μm thick as a core-formingmaterial layer 38 is thus formed on the barium borosilicate glasssubstrate 32. The refractive index of the PMGI layer was measured to be1.540, FIG. 4A.

[2] Next, the core-forming material layer 38 is light-exposed through aphotomask 40 which shadows a core-formative area 38 a of thecore-forming material layer 38.

In this embodiment, a photomask 40 having a linear pattern is set abovethe PMGI layer 38, the pattern being a 10-μm-wide line-and-space forexample. Using the photomask 40, deep-UV light, i.e. an energy beam witha wavelength of 300 nm or shorter, is exposed for 2 hours using anexposure apparatus called mask aligner. The light exposure isgraphically expressed with the arrows in FIG. 4B. PMGI in the exposedarea 38 b becomes soluble to developing solution due to cleavage at theintramolecular bonds.

[3] Next, the core-forming material layer 38 after the light exposure isdipped in the developing solution to leave a portion of the core-formingmaterial layer in the core-formative area 38 a.

More specifically, as continued from the exposure process, developmentprocess is carried out. An aqueous solution of approx. 3 wt %tetraethylammonium hydroxide solution is used as the developingsolution. The substrate 32 together with the PMGI layer 38 are dipped inthis developing solution at 23° C. for 10 minutes. The portion 38 b ofthe PMGI layer exposed by deep-UV light is dissolved and removed, whereresidual PMGI layer 38 a is to be a linear core pattern 34 (also simplyreferred as core) After that, the substrate 32 on which the core 34 isformed is washed with pure water and then dried, FIG. 4C.

An observation of the core 34 by scanning electron microscopy showed thecore 34 being formed in a 5-μm-wide linear form, which is thinner than a10-μm-pitch line-and-space pattern of the photomask 40. This sort ofthinning is ascribable to diffraction of the light into the non-exposingarea 38 a, since a thickness of 5 μm of the PMGI layer 38 is much largerthan that of ordinary photoresist layer, and the exposure time ofdeep-UV light is significantly longer.

It is thus required to adjust the photomask 40 assuming such thinning ofthe post-exposure pattern. The core 34 made of PMGI is thus formed onthe barium borosilicate glass substrate 32.

[4] Next, an overclad 36 is formed so as to cover the core 34.

In this embodiment, an UV setting resin with a refractive index of 1.528(product of NTT-Advanced Technology Corporation, product code No. 8101),which is the same as that of the substrate 32, is selected as a polymerfor composing the overclad 36. The UV setting resin is coated on thesubstrate 32 so as to cover the core 34. Here the coating is carried outat 3,000 rpm for 30 seconds using the spin coating method. The obtained,coated film is exposed with ultraviolet radiation from a high-pressuremercury lamp for 5 to 10 minutes to be hardened. The substrate 32 onwhich the coated film is formed is then annealed at 80° C. for 2 hoursin a furnace conditioned to the atmospheric environment, to completepolymerization reaction of the coated film. An approx. 30-μm-thickoverclad 36 is thus formed over the core 34, FIG. 4D.

In this embodiment, the ratio (n₁-n₂)/n₁ of the difference of therefractive index n₁ of the core 34 from that n₂ of the clad (i.e.substrate 32 and overclad 36) to the refractive index n₁ of the core is0.78%. That is, the ratio of the difference in refractive index of thecore 34 and the clad 32, 36 to the refractive index of the core 34, orthe normalized difference, falls within a range from 0.3 to 3.0%, sothat light can satisfactorily be transmitted based on total reflection.

The obtained laminate is then polished on its both side faces includingthe both end surface of the linear core 34 to complete an opticalwaveguide of 2 cm long, see FIG. 3.

According to this embodiment, obtained is the optical waveguide having anovel structure comprising the glass substrate 32; the core 34 providedon the glass substrate 32; and the overclad 36 provided on the substrate32 so as to cover the core 34 and made of polymer; where the core 34being made of PMGI.

The thus obtained optical waveguide 30 was input with 633 nm light,which is a common wavelength in optical communications, to measuretransmissive attenuation. It was found that a substantially constantvalue (approx. 2.0 dB/cm) was obtained independent of locations.

Although the core 34 was made of PMGI in this embodiment for the case inwhich the optical waveguide 30 comprises the substrate made of inorganicmaterial; the core 34; and the overclad made of polymer; the material isnot limited thereto, and any other material is available for the core 34as far as the ratio of the difference in refractive index between theclad (substrate 32 and overclad 36) and the core 34 to the refractiveindex of the core 34 can fall within a range from 0.3 to 3.0%.

Inorganic material composing the substrate 32 and polymer composing theoverclad 36 can arbitrarily be selected so far as their refractiveindices near ly coincide.

When the core of the optical waveguide is formed using PMGI,constitution of the other parts of the optical waveguide is not limitedto that having the substrate and the overclad, but can be such that theunderclad is formed between the substrate and the core. In such a case,the substrate is not necessarily be made of inorganic material andarbitrary kind of substrate available.

In this embodiment, the core made of PMGI is fabricated by exposing thecore-forming material layer and then by the development process. Thematerial for the core thus fabricated is not limited to PMGI, but can beone having a positive photoresist property, and having a refractiveindex to which the ratio of the difference in refractive index from theclad falls within a range of 0.3 to 3.0%.

The entire disclosure of Japanese patent application Nos. 266422/1998and 239662/1999 filed on Sep. 21, 1998 and Aug. 26, 1999, respectively,including the specifications, claims, accompanying drawings andabstracts of the disclosures are incorporated herein by reference intheir entirety.

While the present invention has been described with reference to theparticular illustrative embodiments, it is not to be restricted by thoseembodiments. It is to be appreciated that those skilled in the art canchange or modify the embodiments without departing from the scope andspirit of the present invention.

What is claimed is:
 1. An optical waveguide comprising a clad and acore, said core being made of polymer material containing a repetitiveunit having either one of formulae (1) and (2):


2. An optical waveguide according to claim 1, wherein said clad is madeof polymethyl methacrylate.
 3. An optical waveguide according to claim1, wherein said clad comprises an underclad provided under said core,and an overclad provided over said underclad for enclosing said coretogether with said underclad.
 4. An optical waveguide comprising a cladand a core, said core being made of polymer material containing arepetitive unit having formula (3):


5. An optical waveguide according to claim 4, wherein said cladcomprises ultra-violet setting resin.
 6. An optical waveguide accordingto claim 4, wherein said clad comprises an underclad provided under saidcore, and an overclad provided over said underclad for enclosing saidcore together with said underclad.
 7. An optical waveguide according toclaim 4, further comprising a substrate made of inorganic material, saidcore being provided on said substrate; said clad comprising an overcladprovided on said substrate to cover said core and made of polymer; saidsubstrate and said overclad having a refractive index (n₂) substantiallyequal to each other, said substrate, said overclad and said core havingrefractive indices defined by a ratio (n₁-n₂)/(n₁) of a difference ofthe refractive index (n₁) of said core from the refractive index (n₂) ofsaid substrate or said overclad to the refractive index (n₁) of saidcore falling within a range from 0.3 to 3.0%.
 8. An optical waveguideaccording to claim 7, wherein said inorganic material is glass.